The present invention relates to a control system for an infinitely variable belt-drive automatic transmission for a motor vehicle, and more particularly to a system for controlling the speed of changing the transmission ratio in accordance with driving conditions of the vehicle.
A known control system for an infinitely variable belt-drive transmission comprises an endless belt running over a drive pulley and a driven pulley. Each pulley comprises a movable conical disc which is axially moved by a fluid operated servo device so as to vary the running diameter of the belt on the pulleys in dependency on driving conditions. The system is provided with a line pressure control valve and a transmission ratio control valve. Each valve comprises a spool to control the oil supplied to the servo devices.
The transmission ratio control valve operates to determine the transmission ratio in accordance with the opening degree of a throttle valve of an engine and the speed of the engine. The line pressure control valve is adapted to control the line pressure in accordance with the transmission ratio and the engine speed. The output of the engine is transmitted to the drive pulley trough a clutch such as an electromagnetic clutch. The line pressure is controlled to prevent the belt from slipping on the pulleys in order to transmit the output of the engine.
When starting the vehicle, the transmission ratio is set at a maximum value. When the vehicle speed and engine speed reach set values under a driving condition, the transmission ratio starts to change (to upshift). At that time if the engine speed is kept constant, the transmission ratio is automatically and continuously reduced at a speed which is determined by line pressure, the pressure of oil supplied to the servo device of the drive pulley, and the actual transmission ratio. In such a system, the speed of changing of transmission ratio up to a desired transmission ratio can not be controlled in accordance with driving conditions. Accordingly, hunting or overshooting of the transmission ratio occurs, which decreases the driveability of the vehicle.
Japanese Patent Laid Open No. 59-208255 discloses a system for controlling the transmission ratio changing speed. In the system, the transmission ratio changing speed (downshifting speed) is increased when a brake pedal is depressed. However, the speed is constant irrespective of the degree of the pedal depression. Accordingly, the control operation does not effect proper engine braking, causing excessive or deficient engine braking.